The Journal of
Physical Chemistry
0 Copyright, 1988, by the American Chemical Society
VOLUME 92, NUMBER 15 JULY 28,1988
LETTERS Hole Burning of Porphines Fixed in a Polymerized Lipid Bilayer Membrane Eishun Tsuchida,* Hiroyuki Ohno, Michinori Nishikawa, Department of Polymer Chemistry, Waseda University, Tokyo 160, Japan
Hiroaki Hiratsuka, Koichi Arishima, and Toshiyuki Shimada NTT Opto-electronics Laboratories. Ibaraki 31 9-1 1 , Japan (Received: January 19, 1988; In Final Form: May 27, 1988)
Porphine derivatives containing four alkyl chains with unpolymerizable or polymerizable hydrophilic end groups in peripheral positions were incorporated into polymerizablelipid bilayers. Hole burning of the porphine derivative which was copolymerized in the liposome environment required much smaller energies for the hole-burning photochemistry as compared to monomolecularly dispersed porphine in a polymerized liposome environment. The distribution of the nonpolymerizable porphine was changed upon the polymerization of the lipid matrix, whereas the polymerizable porphine guest molecule maintained its dispersed state. This is, to our knowledge, the first system that allows the preparation of very highly concentrated host-guest systems (>lo-* mol/L) suitable for hole burning.
Introduction ~~l~ burning is known to be a special kind of saturation spectroscopy in the optical domain, having many analogies with N M R spectroscopic measurements.' Various applications of the method have been suggested, for example, optical frequency multiplexing schemes for data storage: as well as high-resolution optical studies of large biomolecular assemblies with small probe molecule^.^ So far the reported hole-burning experiments were limited to very low concentrations of guest molecules (1 04-10-5 mol/L). Much effort is being made to increase the concentration of guest molecules in the various matrices; a higher concentrations of guest molecules will for instance increase the ultimate storage capacity of an optical hole-burning memory. The system we chose are lipid bilayer membranes containing amphiphilic porphines in high concentrations as molecularly doped or copolymerized guest *To whom correspondence should be addressed.
molecules. Some of the authors have reported that the dispersion state of unpolymerized molecules changes after polymerization Of their environment* In this letter, the authors report, for the first time, the holeburning property of lipid bilayer membranes containing amphiPhilic PorPhines as guest molecules in high concentration.
Experimental Section Polymerizable amphiphiles 1- [9-(p-vinylbenzoyl)nonanoyl~-20-octadecyl-ruc-glycero-3-phosphocholine (2) and 1,2-bis(2,4octadecadienoyl)-sn-glycero-3-phosphocholine(3) (see Figure 1) are used as matrix molecules which are known to form stable ( 1 ) Friedrich, J.; Haarer, D. Angew. Chem., Int. Ed. Engl. 1984, 23, 113. (2) Gutierrez, A. R.; Friedrich, J.; Haarer, D.; Wolfrum, H. IBM J . Res. Dev. 1982, 26, 198. ( 3 ) Friedrich, J.; Scheer, H.; Zickendrant-Wendelstadt, B.; Haarer, D. J . Am. Chem. SOC.1981, 203, 1030; J . Chem. Phys. 1981, 74, 2260.
0022-3654/88/2092-4255$01.50/0 0 1988 American Chemical Society
4256 The Journal of Physical Chemistry, Vol. 92, No. 15, 1988 C%=CH-@CO(C%)8COO?-$ CH3(CH2)1707H 0 CH20r0CH2CH2N(CH3)3
+
Letters
CH3(CH2)12CK-CH-CH~CHCOOCH2
I
r e
+
CH.$k$)12CH=CH-CH=CHCOOCH c~o~OcH2Ck$N(CH3),
0-
0-
3
2
OH
I
OH
on
la
lb
Figure 1. Structural formulas of host and guest molecules: la, lb, porphine derivatives; 2, 1- [9-(p-vinylbenzoyl)nonanoyl]-2-Ooctadecyl-racglycero-3-phosphocholine;3, 1,2-bis(2,4-octadecadienoyl)-sn-glycero-3-phosphocho~ine.
bilayer structures in aqueous media.4p5 The guest molecules l a and lb, are synthesized in our laboratory, and preparative methods have already been reported Especially, as l b has a polymerizable itaconyl group on the hydrophilic end of every alkyl chain, it can be copolymerized with 3 in the mixed liposome.’ Lipids and porphines with a molar ratio of 50 to 1 were homogeneously dissolved in dehydrated chloroform. Their liposomes were prepared with the same method as reported pre~iously.~ The polymerization of the above liposomes was carried out by UV irradiation or by radical cleavage of azobis(2-amidinopropane) dihydrochloride (AAPD) ,* Liposome suspensions, which were prepared in the way described above, were cast on quartz substrates and dried. Since no spectral change in UV/vis absorption was observed before and after the casting p r d u r e , the chemical environment of porphine in the liposomal suspension was considered to be maintained. The samples were mounted in a cryostat and slowly cooled down to 4.5 K. Spectral holes were burnt in the Q,(O,O) absorption band for porphine by an Ar pumped dye laser.9 The burnt holes were detected with a 1m-monochromator.l0
Results and Discussion Figure 2a shows the low-temperature (4.5 K) absorption spectrum of the unpolymerized host-guest system l a in 2. Holes could be easily burnt in the neighborhood of 639 nm by laser irradiation (3 mW/cm2 X 60 s) at 4.5 K; no hole could be burnt (4)Ohno, H.; Ogata, Y.; Tsuchida, E. J . Polym. Sci., Polym. Chem. Ed. 1986,24, 2959. (5) Ohno, H.; Ogata, Y . ;Tsuchida, E. Macromolecules 1987,20, 929. (6) Matsmhita, Y.;Hasegawa, E.; Eshima, K.;Tsuchida, E. Chem. Lett. 1983, 1387. (7) Nishide, H.; Yuasa, M.;Hashimoto, Y.; Tsuchida, E. Macromolecules 1987,20, 459. The complete copolymerization of porphine derivatives and lipids was confmed with gel permeation chromatography,ultracentrifugation, and transmission electron microscopy. (8) Polymerization of 3 gave only oligomers when initiated by UV irradiation. A radical polymerization of 3 is effective to prepare more stable polymerized liposomes. Details are mentioned in ref 5. (9)Single-frequencyAr pumped dye laser, Spectra Physics 380D, has the cm-l and is focused on the sample (0.25 cm2). output line width of (10) The lm-monochromator, Nippon Bunko CT-100,has the resolution of 0.4cm-l and the grating of 1800 lines/”.
0.1
0
0.2 h
L
: 0.1 C
m
.-
c)
P
0
a 0.2
0.1
C
E
0
I
I
I
I
630
640
650
660
Wavelength
6 D
(nm)
Figure 2. Hole burning in the Q,(O,O) absorption bands of porphines for porphine/lipid systems at 4.5 K: (a) l a in 2, 3 mW/cmz X 60 s; (b) l a in poly2, 15 mW/cmz X 720 s; (c) lb/3 copolymer, 1 mW/cm2 X 30 s (see text).
at 651 nm under the same conditions (Figure 2a). Since the holes could be easily burnt at 639 nm for unpolymerized samples, it was assumed that hole burning occurred in the Ql(0,O) absorption band of the homogeneously dispersed porphine molecules. On the other hand, the shoulder at 651 nm is considered to reflect
Letters the Q,(O,O) band of highly aggregated porphines." A polymerization of the systems with unsaturated bonds (lb, 2,3)can be carried out by UV irradiation. An example is shown in Figure 2b, where a matrix consisting of molecule 2 was photopolymerized containing l a as guest molecules. Upon polymerization the Q,(O,O) band was shifted from 639 to 654 nm with a simultaneous increase in absorption intensity (Figure 2b). Similar absorption shifts were reported for pendant-type porphyrins on polymer backbones.12 As far as the hole-burning properties of the system of a polymerizable host and unpolymerizable guest are concerned, considerably higher laser intensities had to be used (15 mW/cm* X 720 s). This hole-burning energy is 60 times higher as compared to the intensities used for the unpolymerized sample. As the third sample, the polymerizable amphiphile 3 as membrane matrix was copolymerized with the polymerizable porphine l b by the addition of AAPD.5 The spectral characteristics before and after polymerization were completely identical (Figure 2c). As far as the hole-burning characteristics of this sample are concerned, there is no measurable change in the hole-burning properties before and after the polymerization. It is interesting to note that the hole-burning efficiency is very high compared to the sample of Figure 2b and slightly higher than the sample of Figure 2a. The decrease of the hole-burning efficiency in the polymerized sample comparing unpolymerizable porphine (la in poly2; Figure 2b) is interpreted as being due to energy transfer between aggregated guest molecules. The aggregation of unpolymerizable molecules is known to be induced by the polymerization of a polymerizable matrix." This aggregation of chromophore dye molecules may be the most serious drawback for the organic hole-burning memory system. Many hole-burning experiments are limited to very low concentrations of guest molecules (iO-4-10-~ mol/L). A great deal of effort is being made to increase the concentrations of guest molecules in various matrices. In the latter, we show that, using dye-doped liposome as host-guest system, we obtained the very highly concentrated (>lo-* mol/L) hole(1 1) It is commonly believed that a phase separation occurs as a result of polymerization of liposomal membrane, in which both polymerizable and unpolymerizable amphiphiles are coexistent. Because a polymerization process requires the formation of covalent bonds between polymerizable components, the polymerization process can initiate a phase separation. This may induce a phase separation of the unpolymerizable component of the liposomal membrane. The phase-separated liposome structure (frames) can be seen by scanning electron microscopy (e.g., Tsuchida, E., et al. Polyym. Bull. 1985,14, 487). (12) Kamachi, M.; Cheng, X.S.; Kida, T.; Kajiwara, A.; Shibasaka, M.; Nagata, S. Macromolecules 1987,20,2665.
The Journal of Physical Chemistry, Vol. 92, No. 15, 1988 4251
8
641
642
Wavelength
643
(nm)
Figure 3. Multiple hole burning for lb/3 copolymer system at 4.8 K recorded as a difference spectrum with the laser radiation of 1 mW/cm2 X 5 min (see text).
burning materials. Our spectroscopic experiments (Figure 2c) indicate that the local environment around the polymerizable porphine in a lipid bilayer is not changed by the copolymerization process. This suggests that the porphine derivatives are homogeneously distributed in amphiphiles without measurable aggregation. To analyze hole-burning effects on the neighboring holes which are close in energy space, hole burning was carried out with a sequence of holes between 641.729 and 642.715 nm as shown in Figure 3. Eight holes were burnt in the following sequence: 1, 641.915; 2,642.018; 3,642.115; 4,642.216; 5,642.315; 6,642.515; 7, 641.729; 8, 642.715 (nm). It was clearly found that the depth of previously burnt holes was reduced considerably by the subsequent hole burning at different wavelengths. There was no measurable wavelength dependence on this reduction of holes. This result suggests that the average hole-filling phenomena are mainly caused by the wide absorption spectrum of the hole-burning products. In conclusion, the authors have documented the possibility of data storage in highly concentrated chromophore systems with hole-burning experiments on porphines covalently bound in polymerized bilayer membranes. Based on the above encouraging first result, the present experiments will be carried out in more detail and results will be presented in the near future.
